Methods and apparatus for reducing the sound level of a gearbox

ABSTRACT

The present invention is directed to improved methods of reducing the sound level caused by meshing of gear teeth in a gear box, and in particular by reducing distortion caused by heat treatment as used in the manufacturing process of the gear teeth.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates generally to sound reduction of meshing mechanical parts, and more particularly to means for reducing the sound level of the gear box caused by the meshing of gear teeth, and yet more particularly by means including reducing the distortion of the gear teeth caused by heat treatment.

[0003] 2. Description of the Related Art

[0004] In the prior art, various mechanical devices which include meshing parts, such as gear boxes, have had substantial noise levels, and also in some instances a high-pitched whine which, separately or together, have caused certain difficulties in the work place related to the hearing of workers. Yet additionally, in certain instances additional expense and/or dislocation, as well as down time, have occurred because of such inordinate levels of sound and/or whine. In some instances, equipment to mask or reduce the levels of sound has been rendered necessary, such as high-tech ear protection devices, etc.

[0005] In the prior art, substantial difficulties in reducing the sound level of such meshing gears have been encountered due in material part to the difficulty in discovering the specific cause of such problem. Whereupon, as a material part of the present invention, the Applicant hereof has determined that in substantial part one material reason for such excess noise levels in the meshing of gears (such as, for example, for use in the assignee's gear boxes) has been distortion of the gear teeth in the manufacturing process. More particularly, it has been discovered and determined by the Applicant that heat distortion of the gear teeth in the manufacturing process has been a material element in causing such excess noise levels during meshing of the gears.

[0006] Based upon the research and development of the Applicant in determining the cause for such excess noise levels, it has been a material object of the present invention to substantially reduce the distortion of the gear teeth caused by heat treatment during the manufacturing process.

[0007] In addition to the above material object, other objects of the present invention include the use of improved normalization and annealing techniques in the reduction of such heat distortion in said gear teeth.

[0008] In yet further particulars, gear boxes and related apparatus as used in the lawn and garden industry that are being imported into the European Union are special problems caused by the sound level regulations imposed by the European Union. In addition, refrigeration trucks typically have utilized a compressor that is powered by a gear box mounted on the cab. Accordingly, high levels of noise and whine have rendered riding and sleeping conditions in the cab difficult. Other examples that require quiet running gear boxes abound. The present invention is directed to meeting these needs.

BRIEF SUMMARY OF THE INVENTION

[0009] The present invention is directed to improved methods of reducing the sound level caused by meshing of gear teeth in a gear box, and in particular by reducing distortion caused by heat treatment as used in the manufacturing process of the gear teeth.

[0010] In one particular, hot rolled materials are utilized in some preferred embodiments, and in other preferred embodiments cold rolled materials that have been normalized and annealed are utilized. Moreover, the carburizing process that is utilized is carried out at approximately 1500°-1900° F., followed by tempering of the gears at a temperature level of approximately 375°-600° F.

[0011] The present invention will be better understood with regard to the following detailed description, preferred embodiments, appended drawings, and attached patent claims.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

[0012]FIG. 1 is an external view of one preferred embodiment of gearbox utilizing gears made according to the methods of the present invention.

[0013]FIG. 2 is a cross sectional view of one preferred embodiment of the gearbox taken along axis 2-2 of FIG. 1 and showing the details of the internally journaled input and output shafts on tapered roller bearings.

[0014]FIG. 3 is a view of the input shaft of one preferred embodiment showing placement of an integral gear and two integral tapered roller bearing members.

[0015]FIG. 4 is a view of a tapered roller bearing assembly which can be used in the gearboxes incorporating the gears of the present present invention.

[0016]FIG. 5 is a view of an alternate embodiment of another form of gearbox.

DETAILED DESCRIPTION OF THE INVENTION

[0017] In preferred embodiments of the methods and apparatus of the present invention, decreasing the distortion of the gear teeth caused by heat processing during the manufacturing thereof is significant in reducing the sound level resulting during the meshing of gear teeth, such as, for example, are used in a gear box.

[0018] Referring now to the figures as above described, the present invention provides for methods for manufacturing gears as used in a gearbox. Preferred gearboxes include those which are strong, self-centering, resistant to axial forces, and both versatile and moderate in cost. Gearbox 100 is generally illustrated in FIGS. 1 and 2. The gearbox 100 includes a housing 102, which may preferably be symmetrical and formed of die-cast aluminum or other metal. One skilled in the art will recognize that there are many ways of constructing the housing 102. The methods of the present invention are not limited to gears depicted in the particular examples disclosed in the accompanying Figures, nor to such gearboxes. For example, the housing 102 might instead be comprised of two symmetrical die-cast metal pieces bolted together.

[0019] The housing 102 contains gear shaft openings 104 and 106. Input shaft 108 is disposed through gear shaft opening 104 and output shaft 110 is disposed through gear shaft opening 106. Two input shafts 108 and two output shafts 110 may be utilized in alternative embodiments. The embodiment disclosed in FIGS. 1 and 2 employs a straddle-mount design, which minimizes the size of the gearbox 100.

[0020] The input shaft 108 has an integral first gear 112, an integral first tapered roller bearing cone 114, and an integral second tapered roller bearing cone 116. The housing 102 has a complimentary first tapered roller bearing cup 118 and a complimentary second tapered roller bearing cup 120. The first tapered roller bearing cone 114 is disposed to engage the first complimentary tapered roller bearing cup 118 and the second tapered roller bearing cone 116 is disposed to engage the second complimentary tapered roller bearing cup 120 to rotationally journal the input shaft 108. Such tapered roller bearing components may be interchanged. Also ball bearings can be used, or a combination of ball bearings and roller bearings.

[0021] The output shaft 110 has an integral second gear 122, an integral third tapered roller bearing cone 124, and an integral fourth tapered roller bearing cone 126. The housing 102 has a complimentary third tapered roller bearing cup 128 and a complimentary fourth tapered roller bearing cup 130. The third tapered roller bearing cone 124 is disposed to engage the third complimentary tapered roller bearing cup 128 and the fourth tapered roller bearing cone 126 is disposed to engage the fourth complimentary tapered roller bearing cup 130 to rotationally journal the output shaft 110. The output shaft 110 may extend through the housing 102 to permit the mounting of a clutch field 132 on the output shaft 110. The housing 102 may also have mounting holes 134 for mounting external components, such as the clutch field 132.

[0022] The first gear 112 is coupled to the second gear 122 such that a rotation of the input shaft 108 results in a rotation of the output shaft 110. One skilled in the art will recognize that the first gear 112 may be directly or indirectly coupled to the second gear 122. The input shaft 108 may function as the output shaft 110, and vice versa.

[0023] The use of integral gears 112, 122 and integral tapered roller bearing cones 114, 116, 124, 126 provides for a stronger gearbox 100 with closer tolerances than prior art gearboxes. Thus each gearbox 100 can be consistent one to the next and the input shaft 108 and output shaft 110 can be stabilized with respect to the gear shaft openings 104, 106 on the housing 102. This eliminates the need for shims when mounting other components, such as the clutch field 132, on the outside of the gearbox 100. The use of tapered roller bearings (i.e. first taper roller bearing cone 114 engaging complimentary first tapered roller bearing cup 118) to rotationally journal the input shaft 108 and the output shaft 110 also eliminates concentricity concerns as the bearings are self-centering. Tapered roller bearings are also stronger and more resistant to axial forces than standard ball bearings. Thus, external components, such as the clutch field 132, may be press-mounted on the output shaft 110 with a significantly reduced likelihood of damage to the bearings.

[0024]FIG. 3 is a view of the output shaft 110 of one preferred embodiment showing placement of the second integral gear 122, the third integral tapered roller bearing cone 124 and the fourth integral tapered roller bearing cone 126. The second integral gear 122, third integral tapered roller bearing cone 124 and fourth integral tapered roller bearing cone 126 can be affixed to the output shaft 110 through the use of fixed stops 136. Other means can be used to affix the second integral gear 122, the third integral tapered roller bearing cone 124 and the fourth integral tapered roller bearing cone 126 to the output shaft 110. For example, welding techniques may be used. Alternatively, the output shaft 110 and the integral components, such as the second integral gear 122, may be composed of a single piece of material.

[0025]FIG. 4 is a view of a tapered roller bearing assembly 150 which can be used in a preferred embodiment of the present invention. The tapered roller bearing assembly consists of an inner race 152, a cage 154, rollers 156 and an outer race 158, or cup. The inner race 152, cage 154 and rollers 156 together constitute an inner race assembly 160, or cone.

[0026] Although FIGS. 1 and 2 depict one form of applicant's gearbox 100 with input shaft 108 and output shaft 110 disposed at an angle of 90° to each other, it should be understood that other angles of disposition may also be utilized. For example, FIG. 5 is a view of an alternate embodiment of the improved gearbox 100 of the present invention with the input shaft 108 disposed at an angle of 135° with respect to the output shaft 110.

[0027] In summary, the inventive methods of the present invention are utilized to produce gears utilized in the above described gearboxes, and decrease materially the sound level emitted during operation of the gearbox.

[0028] Several different embodiments of the methods of the present invention have been preferred. In particular, the discovery that excess sound has been caused by heat distortion of the gear teeth has been remedied, in material part, by the use of hot rolled materials for such gear bodies. Additionally, in other preferred embodiments when cold rolled materials are utilized, those materials are normalized.

[0029] The applicant for the present invention has further conducted certain experiments with the heat treater to design gear blanks that would greatly reduce or eliminate the heat treat distortion of the gear teeth. These experiments may be classified under four categories: (1) materials, (2) design, (3) reduction of heat distortion, and (4) uniformity of case hardness depth.

[0030] (1) Materials

[0031] Some of the distortions seen by those skilled in the art of heat treating appear to be caused by variations in material. Cold rolled materials are known to include residual stresses not found in hot rolled materials. Another reason is that there are impurities in steel that are not listed in chemical analysis. For example, lead is sometimes added to steel to aid in machining.

[0032] If cold rolled materials are to be used, it has been found that they should be normalized and annealed to reduce residual stress. Steels from the same mill lot will have similar impurities and will react the same in heat treat. In low carbon steels such as 8620, lead is not needed at the speeds and feeds as machined by applicant. When leaded steel is heated, the lead cooks out leaving voids where it was interspersed, thus lowering the steel's potential hardness.

[0033] Additionally, oxidation is present on the outside of hot rolled steel. To avoid the effects of the oxidized layer, a ¼ inch of material should preferably be removed from the diameter of the bar. Taking this into account the number of different sizes of material used can be reduced. Material certification and jominy (hardinability test) may also be provided, thus aiding further in determining the correct heat treating settings for the material.

[0034] (2) Design

[0035] Gear blank should be designed for use in the heat treatment process. For example, in the carburizing process, steel is heated to a critical temperature in an atmosphere rich in carbon. When the steel reaches this temperature, it begins to absorb carbon. Because thinner sections will heat faster, they began absorbing carbon sooner, allowing a deeper case depth in this area (see discussion below). It is important that the blueprints for such gears show where the case depth is to be measured.

[0036] Thin cross-section, such as hubs on gears, will react to quenching differently than the thicker section of the gear, causing warping, making one end of the bore smaller than the other. Hence, making the hubs as large as possible will minimize such distortion, with the added resultant benefit of making gear blanks easier to hold in the lathe operations.

[0037] It has been found that fine pitch gears will nearly always harden through the entire tooth. If fine pitches are needed, material designed to through-harden, such as 4140, 4150 and some stainless steels, should preferably be used. Additionally, when heat treating specifications are changed on a relevant blueprint, this information shall be noted on the purchase order when the gears are sent to the heat treater.

[0038] Yet further, it has been determined that some of the run out experienced during assembly comes from the placement of the part number. Many gears do not have room to move the imprint away from the bearing areas. Hence, it must be decided whether the part number is necessary, and if so, method to place a part number without displacing material must further be used.

[0039] (3) Reduction of Heat Distortion

[0040] It has also been determined that when the gear is heated, residual stress will relax causing distortion in the part. Residual stress can come with the material, as in cold rolled bars, or can be a side-effect of machining. Stress from machining can be reduced by keeping tools sharp through the entire process.

[0041] To further reduce residual stress, gear blanks can be normalized before cutting teeth. This process would double the time and cost of heat treating, and require additional machining after normalization to remove distortion.

[0042] Some of the distortion from hardening can also be reduced by adding a step to the heat treating process. In one process now used, gears are carburizing and cooled to quenching temperature, then quenched all in the same run of the furnace. By carburizing and then allowing them to cool to room temperature, thereafter reheated to quench temperature, and finally quenching, it has been found that there will be less distortion. This process would add a half a day to the time required to heat treat, and would add approximately 70% to the cost.

[0043] If the material properties are consistent, a pair of gears can be heat treated, and thereafter changes can be made to the gear tooth profile to compensate for distortion in the tooth, such as spiral unwrap. This would mean that Gleason gear machines, for example, would remain idle after initial set up until the sample gears returned from heat treat. This step would have to be repeated whenever there had been a change in material, such as steel from a different mill lot.

[0044] (4) More Uniform Case Hardness Depth on Teeth

[0045] To be certain a gear tooth is hardened to the correct hardness and case depth, the blueprints specify the precise location where the case depth is to be measured.

[0046] Of course, measuring the case depth is inherently a destructive test. In particular, a cross-section of material is measured on a hardness tester to find where the case hardening ends. If a gear blank is not available, a sample plug is preferably heat treated with the batch of gears. To ensure the proper case hardening depth, a scrap gear should be sent with every batch of gears to heat treat. The tooth from this scrap gear can then be cross-sectioned to get a more accurate measurement of the case depth.

[0047] The carburizing process, as used in connection with certain preferred embodiments of the present invention, has three stages: carburizing, quenching, and tempering. In the tempering stage, the hardness is drawn back to the desired hardness. In the present invention, carburizing and quenching are done at high temperatures, between 1500°-1900° F. Tempering is done at a relatively low temperature, between 375°-600° F. Moreover, the tempering temperature of our fine pitch gears is below 500° F. If gears are heated to a temperature of the tempering temperature, they will become softer. The temperature to which the gears are heated for shrink fits is measured to assure this temperature is not exceeded.

[0048] The face width is generally made wider than standard design practices. Lengthening the face width from standard dimensions increases the face contact ratio. The higher the face contact ratio, the quieter the gears would be.

[0049] In some further embodiments, minor adjustments are also preferably made to the spiral angle to achieve the quietest gear. Also, the bearing preloads are adjusted for the quietest running and assembly ease. Furthermore, gear dimensions are designed to closer tolerances. Changes to profile mismatch and tooth contact length are made at the time the gears are developed. For example, gear assemblies could be made with zero tolerance and no deflection, the contact on the gear tooth would run from the bottom of the tooth (root) to the top of the tooth (flank). But every component in the assembly has been shown to have some allowance for variation, so it is not practical to design a gear tooth with this full contact. For this reason, a separation in the contact between the root of the tooth and the flank of its mate is preferably figured into the machine setup summary. This separation is called “profile mismatch.” Gear tooth contact at the flank of the tooth adds to the noise level of the gear set by producing a high pitch sound. Part of the gear development process of the present invention is to add profile mismatch to the gear set to ensure that contact is away from the flank in the tooth. If contact is too close to the flank, a new machine summary is generated to move the contact away. To a certain degree this can be done with the same cutters as the original summary, but as a height of the profile decreases, the length of the profile increases. For this reason, it may be necessary to change one or both of the cutters to decrease the contact length and keep the desired profile mismatch. With a wide selection of spiral bevel cutters, we can choose the best tooth contact for any given gear set.

[0050] Only Solid Cutters are to be Used.

[0051] For example, as a result of the techniques and methods of the present invention, noise levels have been reduced to 77 dB on the model 200M 1:1 ratio gear box of the Curtis Machine Company, of Dodge City, Kans., with no whine. On the Curtis model 200C, the noise level on the 1:1 gear set has been reduced to 77 dB with no whine, and on the 1.4:1 ratio speed up to 83 dB with no whine. On the Curtis model 215, the noise level has been reduced on the 1:1 ratio to 81 dB with no whine.

[0052] In conclusion, future gearboxes as used by those skilled in the art will run quietly with substantially reduced or no whine. These improvements are extremely important to the lawn and garden industry that exports into the European Union.

[0053] Additionally, refrigeration trucks normally have the compressor that is powered by a gearbox mounted on the cab. Thus, the present invention provides the drivers with a much quieter ride and sleeping conditions. These beneficial results are but a few examples of the great utility of the present invention.

[0054] From the foregoing it will be appreciated that, although specific embodiments of the methods of the invention, and examples of suitable gearboxes, have been described herein for purposes of illustration, various modifications may be made without deviating from the spirit and scope of the invention. Accordingly, the invention is not to be limited thereby. 

1. A method of reducing the sound level caused by the meshing of gear teeth in a gear box, said method comprising reducing heat treatment distortion of the gear teeth.
 2. The method claim 1 wherein heat treatment distortion of the gear teeth is reduced by use of hot rolled materials.
 3. The method of claim 1 wherein heat treatment distortion of the gear teeth is reduced by use of cold rolled materials which have further been normalized and annealed.
 4. The method of claim 2 further comprising removal of an outside layer of oxidation from said hot rolled steel.
 5. The method of claim 4 wherein said outside layer of oxidation comprises approximately ¼ inch.
 6. The method of claim 1, further comprising carburizing said gears.
 7. The method of claim 6 wherein carburizing comprises heating said gears to a pre-selected carburizing temperature in the presence of gaseous carbon, absorbing said gaseous carbon into the surface of said gears, thereafter quenching said gears, and subsequently tempering said gears.
 8. The method of claim 7 wherein said gears include hubs, and wherein the hubs of said gears are increased in thickness to act more uniformly during quenching.
 9. The method of claim 7 comprising reducing residual stress prior to heat treating said gears.
 10. The method of claim 9 wherein said gears are formed by machining tools and wherein residual stress is reduced by maintaining the sharpness of said machining tools during gear machining.
 11. The method of claim 9 wherein said gears are normalized to reduce residual distortion prior to machining.
 12. The method of claim 7 wherein said gears are heated to said pre-selected carburizing temperature, cooled to room temperature,
 13. The method of claim 6 further comprising establishing standards for the hardness of and case depth of said carborized gears.
 14. The method of claim 13 further comprising testing said hardness and case depth thereof after carburizing said gears.
 15. The method of claim 7 wherein said heating during carburizing of said gears is carried out at approximately 1500°-1900° F.
 16. The method of claim 7 wherein said tempering is carried out at approximately 375°-600° F.
 17. The method of claim 16 wherein said gears are tempered at temperatures below approximately 500° F.
 18. The method of claim 1 wherein the sound level is reduced to less than approximately 77 dB.
 19. The method of claim 1 wherein the sound level is reduced to less than approximately 83 dB.
 20. The method of claim 1, further comprising the substantial reduction or elimination of gear whine and wherein the gears of said gearbox are disposed in profile mismatch array between contacting gear teeth to reduce gear whine.
 21. The method of claim 20 wherein said gear teeth include a tooth root, a tooth bottom, and a tooth flank at the top of said gear tooth, and separation is maintained between said tooth root and said tooth flank to reduce gear whine.
 22. The method of claim 20 wherein said gear whine is substantially eliminated. 